A key problem associated with the measurement of the electromagnetic field inside an electrically large and relatively lossless enclosure is that the field is random and highly variable (with field variations from point to point inside the enclosure). However, the field is also considered to be statistically uniform, for example, the average measured in one general location of the enclosure will be approximately the same as the average measured in another general location within the same enclosure. Because the field is random and highly variable, it is preferable to use a statistical measurement approach to measuring electromagnetic characteristics of an enclosure (e.g., average field level, peak field level, statistical distribution of field levels).
As field mapping locations inside the enclosure would be too time consuming or too equipment intensive, mode stir methods have been widely used for statistical measurement of electromagnetic characteristics of an enclosure. Mechanical mode stirring is the most widely accepted and commonly practiced method of exciting or randomizing the modes in a reverberation chamber to generate a statistically uniform field. Mechanical stir (MS) methods physically alter the boundary conditions within the enclosure to change the distribution of field strengths. This is typically done by the use of an electrically conductive paddle wheel that is rotated at small angular increments. The field is measured at each angular position of the paddle wheel and then the average field strength is determined by averaging over all the measured values obtained through one complete revolution of the paddle wheel.
Mechanical stirring suffers from being time intensive and limited by how well the paddle wheel is designed and used to stir the field. The paddle wheel also adds a level of complexity to the test setup, since its rotation must be synchronized with the measurement equipment and any other paddle wheels used in the test setup.
As an alternative to mechanical mode stirring, frequency stir methods simulate boundary condition variations by changing the wavelength of the excitation source through small increments in excitation frequency. Frequency stirring has traditionally been done by superimposing a band-limited noise signal onto the excitation source which is a single frequency, continuous wave (CW) signal, and then measuring the total power within the bandwidth of the noise signal. By this method, the average field level obtained is taken as the average over all enclosure eigenmodes contained within the measurement bandwidth.
The traditional frequency stir method has limitations associated with the loss of statistical data components and the complexity and performance of the measurement equipment. Because the measurement being made for this method is total power within the noise bandwidth, only the average field level across the noise bandwidth can be determined and the individual field levels that contribute to the total power are not measured. As a result, a statistical analysis of the contributing field levels cannot be made to check the measurement against prevailing mode stir theory. Also, since the excitation source for the traditional frequency stir method is a band-limited noise signal superimposed on a single frequency sine wave, the test equipment setup is more complex, requiring more pieces of equipment including a separate signal generator and receiver. This makes it more difficult to move the measurement equipment for remote area tests. Further, the actual generation of a well defined band-limited noise signal can be problematic (especially at higher frequencies, i.e., above 10 GHz). Since the measurement is a broadband measurement, this technique suffers more than the others from dynamic range limitations and possible noise interference (when used at open area test sites).